Scalable Architecture for Dark Photon Searches: Superconducting-Qubit Proof of Principle

The dark photon is a well-motivated dark matter candidate that appears in many extensions of the standard model. A fundamental mass-range-sensitivity dilemma is always haunting the dark photon search experiments: resonant haloscopes have excellent sensitivity but are narrowband, while nonresonant haloscopes are broadband but not as sensitive as the resonant ones. A scalable architecture integrating numerous resonant haloscopes would be a desirable solution to this dilemma. However, even the concept of a scalable search remains rarely explored due to the size limitation of conventional haloscopes imposed by the dark photon wavelength. Here, we propose and demonstrate a novel architecture using superconducting qubits as subwavelength haloscope units. By virtue of the scalability of superconducting qubits, it is possible to integrate multiple qubits with different frequencies on a chip-scale device. Furthermore, the frequencies of the qubits can be tuned to extend the search mass range. Thus, our scalable architectures allow for sweepable high-sensitivity dark photon searches at multiple frequencies simultaneously. As a proof-of-principle experiment, we designed and fabricated a three-qubit chip and successfully demonstrated a scalable dark-photon search. Our Letter established constraints on dark photons in the mass range of 15.632-15.638, 15.838-15.845, and 16.463-16.468  μeV, simultaneously, and the constraints are much more stringent than the cosmology constraints. In the future, our Letter can be scaled up to boost the scrutiny of new physics.